You don't HAVE to have them. It's nice to have along the way, knowing that you have a Ryhorn with a 25-31 IV in its ATK stat, but most of your stats will be affected by EV training. Of course, though, you don't want a 0 IV in ATK, so I'd suggest using an IV calc when you get your Ryhorn to a certain level (13 should be good enough to roughly see what your IVs are like). If you do this, keep track of your Ryhorn's EVs that it gains in battles (unless you Rare Candy it temperarly), or those EVs could interfere with the IV calc. Getting back on topic here, I wouldn't really worry about the characteristics too much. Just make sure you got a decent nature (adamant in your case) and you should be fine.